From: Joel Mackay <j.mackay@biochem.usyd.edu.au>
  To  : rasmb@alpha.bbri.org
  Date: Wed, 24 Nov 1999 12:56:32 +1100

monomer-dimer-tetramer

Dear all,
I would just like to thank all of those who replied to me a month or two
ago regarding my problem trying to distinguish a monomer-trimer from a
monomer-dimer-tetramer equilibrium (original mail below).
It seems that this is a common problem, and that the best way around it is
to record data over as wide a concentration range as possible. This is
fairly obvious I hear you say, but it was reassuring to hear that
distinguishing these two models is often tricky. I have placed the replies
to this message on my web site, if people want to read them or download
them, go to:

http://www.biochem.usyd.edu.au/~joel/extras/extras.html

I would also just like to say that I think this newsgroup is an extremely
valuable resource for us all (especially for people like myself with only a
few years experience with AU), and I hope it continues on well into the
future.
Thanks again.
Best regards,
Joel Mackay


>Dear all,
>I have a question about deciding which model describes one's data best. I
>have recorded sedimentation equilibrium data at three speeds with three
>different dilutions for a protein which undergoes some self-association. I
>have been fitting the data in NONLIN. If i fit the data by fixing sigma to
>the monomer mass, allowing delta y and lnA values to float, and permitting
>a single association constant to float, i get the best fit with a
>monomer-trimer model (both monomer-dimer and monomer-tetramer have worse
>residuals and higher chi-squared etc according to NONLIN). If I instead
>allow an extra equilibrium constant to float, and call the two associations
>monomer-dimer and monomer-tetramer, i get a slightly lower chi-squared
>(0.0138 vs 0.014 for the monomer-trimer model). My question is, how do i
>decide if the extra complexity of the model is justified. I know there is a
>thing called an F-test, and thought that might be appropriate. If so, how
>does one apply it in this case? What do you all do in these situations? It
>seems that the extra variable is pretty risky, but presumably a
>sufficiantly large reduction in the chi-squared would justify its inclusion.
>cheers and thanks in advance for any help,
>Joel Mackay

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Dr Joel Mackay                                 ph +61-2-9351-3906
ARC Research Fellow                        fax +61-2-9351-4726
Department of Biochemistry
University of Sydney
NSW 2006 Australia
http://www.biochem.usyd.edu.au/~joel/
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